The use of a linearly polarised light is necessary notably for illuminating the liquid crystal valves used in projection or back-projection.
Because of this, there exist on the market numerous types of optical polarisation devices for obtaining a linearly polarised light.
Thus, in the French patent application No. 95 07396 filed in the name of THOMSON Multimedia, corresponding to U.S. Pat. No. 5,900,973, issued May 4, 1999 an optical polarisation device was proposed, affording good recombination of the beams on each other and exhibiting low dispersion.
This optical polarisation device, described in more detail with reference to FIG. 1, is associated with a lamp 1, which can be an arc lamp of the metallic halide type, with a filament or similar. This lamp 1 is positioned at the focus of a reflector 2, of parabolic or elliptical shape, provided in a known fashion with a condenser. The light emitted by the lamp 1 is a white light with random polarisation. It is reflected by the reflector 2 and transmitted onto a polarisation converter assembly. This assembly includes a polarisation separator 3 which consists, in this embodiment, of a stack of glass sheets so as to form an air/glass stack. The polarisation converter also has a mirror 4, whose role is to reflect, towards the polarisation separator 3, the components s of the light beam. This mirror 4 is associated with a .lambda./4 plate 5, which rotates the plane of polarisation of the reflected beam through 90.degree.. At the exit from the polarisation separator 3, there is found a polarisation recombination means 6. In the embodiment in FIG. 1, the means 6 of recombining the polarisation components is a means functioning in total reflection and consists of a transparent sheet produced from a material such as methacrylate, carbonate or other similar materials, glass also being able to be used. This means has, on its face receiving the beam coming from the polarisation separator 3, a set of microprisms 6' placed in a parallel row. The other face is composed of a plane face 6".
In the device described above, the light beam coming from the lamp 1 arrives at the polarisation separator 3. In a known fashion, the polarisation components p of the beam pass directly through the polarisation separator, they arrive on the faces 6' of the recombination means 6 and are then transmitted inside the means 6 so as to leave perpendicularly to the face 6". Moreover, the polarisation components s of the light beam are reflected and sent towards the mirror 4, where they undergo total reflection and pass once again through the .lambda./4 plate 5 so that their polarisation plane is rotated through 90.degree.. The new component referenced p' is then sent onto the polarisation separator 3. At the exit from the separator 3, the component p' is sent onto the recombination means 6, the beam p' strikes the inclined external surface of the microprism at an angle which is almost perpendicular. Next, it is transmitted to the inside of the microprism and strikes the internal face of the adjacent surface on which it is reflected so as to emerge perpendicularly to the face 6".
With this system, a good superimposition of the components p and p', is therefore obtained at the exit. However, the bulk of this optical polarisation device is far from negligible. Moreover, in the plane of the liquid crystal screen which will receive the beams p and p', the two polarised beams have not followed the same optical path. This entails a distribution of illumination which is different for the two beams.